Plant pathogens can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Examples are phytopathogenic fungi the majority of which belong to the Ascomycetes, including the causal agents of powdery mildew of various plants, the Basidiomycetes, including the causal agents of severe rusts of virtually all cereal grains and cultivated grasses and the Deuteromycetes (Fungi imperfecti), including Fusarium, Botrytis and Verticillium. Further pathogens are Oomycetes, which are not true fungi but fungal-like organisms that use the same mechanisms as fungi to infect plants. They include some of the most destructive plant pathogens including Phytophthora infestans, the causal agent of potato late blight and Plasmopara viticola, the causal agent of grapevine downy mildew. Besides fungi, also bacteria including the genii Erwinia, Xanthomonas, Pseudomonas and Raistonia, viruses including Cucumber Mosaic Virus, Barley Yellow Mosaic Virus, Strawberry Mild Yellow Edge Virus, Strawberry Latent Ringspot Virus, Beet Necrotic Yellow Vein Virus and Potato Virus Y, insects including beetles, rootworms, hoppers, locusts, (stem)borers, aphids, mites, thicks, ants, whiteflies, maggots, weevils, midges, caterpillars, butterflies, leaf miners, leaf rolers, bugs, and plasmodiophorid protists including the genii Polymyxa, Plasmodiophora and Spongospora are important plant pathogens.
Plants respond to infection by pathogens by activating their innate immune system. The plant defense systems recognize molecular patterns that are common to many classes of pathogens, e.g. fungal chitin, and may respond to pathogen-specific virulence factors (effectors). Pathogen recognition triggers ion channel gating, oxidative burst, cellular redox changes, protein kinase cascades and other responses that either directly activate cellular changes such as cell wall reinforcement, or activate changes in gene expression that lead to the formation of defensive compounds, e.g. directed to fight infection or to make the plant less attractive to pathogens.
The role of peroxidases in plant protection against pathogens is of the utmost importance. Peroxidases undergo two possible catalytic cycles involving either the consumption or the release of H2O2 and reactive oxygen species. Reactive oxygen species can act directly on pathogens and H2O2 is further known to be a mediator of signal transduction in the establishment of plant defense. Moreover, peroxidases can control the availability of H2O2 in the cell wall, which is a prerequisite for the crosslinking of phenolic groups in the cell wall. The peroxidase mediated crosslinking of different compounds in the cell wall ensures reinforcement of the barrier against pathogen penetration. Peroxidases are also known to be involved in the production of phytoalexins, which are antibiotic compounds produced by plants under stress conditions.
Thus, it is of interest to stimulate the plant immune system in order to control diseases caused by parasites or pathogens such as fungi, oomycetes, bacteria, viruses, nematodes and insects.
Fungal diseases and diseases caused by oomycetes can also be controlled through the use of fungicides in agriculture. Fungicides are chemical compounds or biological substances used to kill or inhibit fungi or oomycetes or their spores. Fungicides sometimes also have an effect on other plant pathogens such as bacteria, viruses, nematodes or insects. A drawback of using certain fungicides is that fungicide residues can be found on food for human consumption, sometimes posing a danger to human or animal health.
Therefore, it is of high-interest in agriculture to improve the performances of chemical fungicides, in particular in terms of biological activity, with the aim of decreasing the amounts of active ingredient to be used.